Mexican Truck Thieves and Cobalt-60, Explained

After news broke of a bizarre truck theft in Mexico, in which the truck was carrying a radioactive source of cobalt-60, media reports said the unlucky thieves were already exposed to a fatal dose. How can that be? Nuclear safety expert and PopMech contributor Andy Karam explains.

As a radiation safety professional, I spend a lot of time trying to convince people that radiation is not as deadly as they fear. While radiation can cause cancer, it's not a very potent carcinogen—certainly not as potent as many industrial solvents. While radiation can cause skin burns or radiation sickness, it takes far more to do so than most people would think, and fatal radiation accidents are relatively rare.

So when I woke up on Wednesday to hear the story that a radioactive source had been stolen in Mexico, my first reaction was not necessarily to worry. In this case, though, I might have underreacted: The thieves who stole a truck carrying a large source of cobalt-60 radiation probably have been exposed to a fatal radiation dose already.

This sample of cobalt-60 came from a Tijuana hospital. Radioactive sources have been used for cancer therapy for decades. In most of the developed world they have been phased out in favor of high-energy x-rays produced by linear accelerators, but much of the developing world continues to use the less finicky radioactive sources. Cobalt-60 is a favored nuclide for therapy because it emits very high-energy gamma rays that are particularly good at penetrating through the body tissues to expose a tumor to therapeutic radiation. However, these radiation sources decay over a period of a few years and a hospital needs to swap them out. Highly trained technicians perform the swap, and the spent source is placed in a well-shielded container for transport.

The most important characteristic of any radioactive source is the amount of radioactivity it contains. More radioactivity means more risk. In the U.S., we measure radioactivity in units called Curies. A cobalt-60 source starts to become dangerous when it reaches an activity of several curies, and sources of as little as 15 curies have caused injuries and even death. The source that the Tijuana hospital was trying to dispose of, and which was in the stolen truck, contained between 2500 and 3000 curies of activity.

With radiation exposure, it's the total dose a person receives that determines the health effects. In the U.S., we measure radiation exposure in units called rem. Humans are exposed to about one-third of one rem every year from natural radiation; a whole-body CT scan will give us about 1-2 rem. At a dose of 100 rem, a person will start to feel ill. By the time they've received 800 or so rem, they will likely die from the exposure.

With cobalt-60, a single-curie source will give you a dose of a little more than 1 rem in an hour if kept at arm's length—about a meter away. But a source the size of the one stolen in Mexico will give a dose of more than 2500 rem in the same time and distance.

When news of the Mexico theft broke, one concern was that the thieves might use it for a "dirty bomb" attack by using the radioactive source to lace a vehicle-borne explosive device. Used in this manner, the cobalt could contaminate a fairly large area. As it turned out, the unfortunate thieves were after the truck—not the source. At some point in the week they realized what they had and removed it from the truck. When they did this, they opened the shipping container and, apparently, the shielding around the source itself. As a result, according to Mexican authorities, they received a whopping dose of radiation—more than a fatal dose—and they are not expected to survive the exposure.

The Mexican authorities have located and are guarding the source to keep it from being stolen again. The plan is to return it to the storage and shipping containers and to ship it to Mexico's radioactive waste disposal facility—where it was headed to begin with.